Heat treatment of cast iron



May 14, 1940.

E. L. BARTHOLOMEW ET AL HEAT TREATMENT OF CAST IRON Filed May 15, 1937 2 Sheets-Sheet 1 2 Sheets-Sheet 2 Filed May 13. 1957 25Z`5r//m/v ENCH//VG- E. L. BARTHOLOMEW ET AL HEAT TREATMENT 0F CAST IRON QJUJ O//vff/JPLfc/Nf/y May 14, 1940.

Patented May 14, 1940 UNITED STATES PATENT OFFICE Marblehead,

Mass., assignors to United Shoe Machinery Corporation, ton, N. J., a corporation Borough of Flemingof New Jersey Application May 13.1937, Serial No. .142,446

4 claims.

This invention relates to the heat treatment of ferrous material and is herein illustrated-in connection with a method of heat treating cast iron in order to impart to itA desiredphysical properties.

It is an object of our invention to provide a method of heat treating cast iron suitable, for example, for cams or other machine parts to produce structures which are especially resistant to wear. In carrying out the invention, iron castings are heated to a temperature above the critical range of the metal and are quenched at a temperature of from 350 F. to 750 F., the iron castings being maintained in a quenching bath for a period of about 15 to 30 minutes. It has been found that this treatment provides a product which is tough, relatively ductile, and which has comparatively. high tensile strength. Cast iron treated in this manner has great wear resist- .ng properties and is particularly adapted for use n cams, gears, and other machine parts which tre subjected to rolling or sliding contacts. 'I'he excellent wear resisting properties are believed to esult from the fact that the crystalline structure f such iron contains a considerable amount of etained austenite which is softer than. but much ougher and more ductile than, its decomposition -roducts. When machine Aparts such as cams omposed of this iron are run in, the ductility of he austenite permits such rearrangement and adustment of the microscopic crystalline structure f the surface portions of the parts as is necesiry to obtain well tting smoothly operating iachine elements. 'Ihe running-in effects a old-.working operation 6i the` material which reaks down the austenite into its decomposition roducts. The resulting structure is a nely di'- ided crystalline aggregate containing one orA lore of the forms known as martensite, sorbite, -oostite, and pearlite, and is appreoiably harder ianbefore the cold Working aswell as being ighly resistant to wear. Parts heat treated in 1e manner above set forth, when subjected to ear tests, have been found to stand up many mes as long as similar parts machined as cast 1d heat treated in accordance with the methods aretofore employed.

In another aspect, the invention provides, as an 'ticle of manufacture, cast iron or iron castgs, producedin accordance with the method `ove outlined, and having a crystalline structure lich has the advantages enumerated above.

The invention, in these and other aspects, is ;closedV in the following specification which ould be read in conjunction with the accompanying drawings, and is pointed out in the claims.

In the drawings: Figs. 1 and 2 are diagrams illustrating the physical properties of one type of cast iron heat treated in accordance with the method of our. invention; and

Figs. 3 and 4 are similar diagrams for another type of cast iron.

In carrying out the method we utilize iron 10 which, as cast, contains 3.0% or more of carbon and which preferably contains also a small amount of nickel. Typical compositions of cast irons which may be employed are as follows:

Castron Percent Percent Silicon 1.42 1.24 Sulphur O. 117 0. 130 Manganese. 0. 37 0. 44 Phosphorus- 0.448 0. 297 Total carbon 3. 36 3. 39 Graphite 2. 50 2. 50 Combined carbon 0.86 0.89 Nickel 1. 52 l. 44 Chromium.. 0.50 Iron Balance Balance The iron casting to be treated isheated above the critical range of the metal, that is, to a temperature above about 1375L7 F. and is maintained at that temperature for from about 15 to 30 minutes, depending upon the size of the casting, after which the casting is quenchedv in a salt or other suitable bath maintained from 350 to 750 F. for a period of from about iiiteen to thirty minutes. The iron is then washed in boiling soda solution, and is permitted to cool to room temperature. A suitable bath vmay be composed of potassium nitrate, 53%, sodium nitrate, 7%, and sodium nitrite, 40%.

We have found that, in most instances, heating above the critical range to about 1550 F. for about 30 minutes gives satisfactory results for 4most castings of small or medium size, but that 45 temperatures as high as 1700 F. maybe employed effectively for large castings. Temperatures as low as about 1375 F. for periods as short as about 15 minutes may be employed for very small castings. f

Heating above the critical range converts the iron of the untreated casting to its austenitic form which, upon quenching, decomposes into other crystalline forms., the amount of conversion of the` austenite depending upon the temperature `55 at a temperature of 35- i2 andenes of the quench and the length of time in the bath. The effect of the hot quench in the range of about 350 to 750 F. is to cause an appreciable amount of austenite to be retained, the amount retained varying from a low percentage at low temperatures of the bath to a high percentage at high temperaturesof the bath. The presence of nickel is believed to assist materially in the retention of austenite, and cast irons containing from 1.0 to 2.5% nickel may be employed with good results.

The resulting hardness of the cast iron increases as the temperature of the bath is lowered, the Brin-ell hardness varying in treated irons of type "A from about 240 up to about 480. This is illustrated by curves in Figs. 1 and 2 in which Brinell hardness numbers and tensile strength are plotted separately against quenching temperatures for a considerable number of heat treated specimens of 0.505 inch and 0.717 inch in diameter, respectively. Figs. 3 and 4 show similar curves for test specimens of type B iron. In type B ironsv the Brinell hardness after treatment varies from about 250 vto 475.

Austenitic cast irons which have a Brinell hardness of over about 350 are exceedingly difficult to machine unless special tools are used. However, it is to be noted that for both type A and type B irons, a high tensile strength is obtained when the quenching temperature is in the neighborhood of 600 F. Whereas the Brinell hardness is less than 350 and, consequently, the iron is machineable. In view of this, for many purposes, it has been found that the iron quenched in a bath of about 600 F. produces excellent results; thatis, the iron, after being heat treated as cast, can be machined, no other heat treatment being required. With irons of a hardness of much over 350 it has been found expedient to machine the castings prior to the heat treatment. This has been done with satisfactory results since such castings have been found to have surprisingly little distortion as a result of the subsequent heat treatment.

The method above outlined is particularly advantageous when employed in connection with castings for cams or gears the surfaces of which, when in use, are subjected to rolling or sliding Contact. When such cams or gears are run in it is found that the material lends itself readily to the fitting or running-in operation Which effects a cold working of the material. At this time the material is toughland ductile, probably due to its retained austente, but during the cold working the-austenite decomposes into a ncly divided crystalline structure which is appreciably harder, and has a higher tensile strength than the material before being worked, and is well adapted to resist wear.

Having described our invention, what we claim as new and desire to secure by Letters Patent of the United States is:

. 1. The method of treating cast iron, containing over 3.0% carbon and from 1.0 to 2.5% nickel, so as to produce a machineable structure of comparatively high tensile strength, which comprises heating the iron to a temperature of about 1550* F., maintaining the iron at that temperature for 30 minutes, then quenching'the iron in a bath maintained at about 600 F. for about 30 minutes, and cooling the iron to room temperature.

2. The method of treating cast iron containing 3.0 to 3.6% carbon, 1.0 to 2.5% silicon, 0.3 to 0.75% manganese, 0.07 to 0.15% sulphur, 1.0 to 2.5% nickel and 0.2 to 0.5% chromium, which comprises heating the iron to a temperature above its critical range for about 15 to 30 minutes, then quenching the iron in a bath maintained at a temperature of about 600 F. for about l5 to 30 minutes, and then cooling the iron to room temperature.

3. The method of treating cast iron containing '3.0to 3.6% carbon, which comprises heating the iron to a temperature above its critical range for about 15 to 30 minutes, then quenching the iron in a bath maintained at a temperature of about 600 F. for about 15 to 30 minutes, and then cooling the iron to room temperature.`

4. The method of treating cast iron castings containing at least 3.0% carbon to obtain a product having anv appreciable amount of austenite retained therein so that upon cold working the austenite decomposes in a finely divided crystalline structure imparting hardness and resistance to wear to the product, which method comprises heating the castings to a temperature above its critical range for a period of time sufficient to provide a predominantly austenitic structure in the iron castings, then quenching the iron castings in a bath maintained at a temperature of from 550 F. to 750 F. for a period of from 15 to 30 minutes, and then cooling the iron to room temperature.

EDWARD L. BARTHOLOMEW. JOHN D. PAINE, JR. 

